1. Field of the Invention
The present invention relates to an automatic focus adjusting apparatus in which light reflected from an object passed through a photographic lens of a camera is received to detect the in-focus state of the photographic lens to carry out focus adjustment. More specifically, the present invention relates to an automatic focus adjusting apparatus which is capable of adjusting focus even if the object is moving.
2. Description of the Related Art
Focus adjusting apparatuses employing phase difference detecting method have been proposed. FIG. 11 is a schematic diagram showing such kind of optical system. Referring to FIG. 11, in the focus detecting apparatus employing the phase difference detecting method, luminous bundle from an object passing through first and second areas of a photographic lens, which areas are in symmetry about an optical axis, are respectively reformed to provide two images. Amount and direction of deviation of the image forming position from the intended focusing position (whether the image forming position is in front of or behind the intended focusing position, that is, whether it is in the front focus state or rear focus state) are known from relative positions of the two images.
The optical system comprises a photographic lens 2, a condenser lens 6 positioned at an intended focal plane 4 behind the photographic lens 2 or further behind the plane, and image reforming lenses 8 and 10 positioned further therebehind. Image sensors 12 and 14 having CCDs as photosensitive elements are arranged on image forming planes of the image reforming lenses 8 and 10. Images on the image sensors 12 and 14 come close to each other near the optical axis 18 when the image of the object to be focused is formed in front of the intended focusing plane, that is, in the so called front focus state. On the contrary, in the so called rear focus state, the images go away from the optical axis 18. In the in-focus state, corresponding two points of the two images are spaced from each other by a prescribed distance defined by the structure of the optical system of the focus detecting apparatus. Therefore, theoretically, the state of focus can be known by detecting the distance between the corresponding two points of the two images.
In an automatic focus adjusting apparatus of a camera containing a focus detecting optical system of the above described type, sequential control is carried out in accordance with a program by a control circuit including a microcomputer, the sequence comprising accumulation of charges corresponding to the light intensity from the object in a CCD image sensor, focus detecting operation (calculation of the amount of defocus) employing outputs from the CCD image sensor, driving of a focusing lens (not shown) of photographic lens 2 in correspondence with the amount of defocus, and stopping of the focusing lens at the in-focus position (releasing of the shutter mechanism . . . when the shutter button is pressed).
The automatic focus adjusting apparatus continuously carries out the above described sequential automatic focus adjusting control when the object image comes near the in-focus state, and carries out continuous AF (automatic focus adjusting) operation, so that exact in-focus position can be set.
The above described automatic focus adjusting apparatus exhibits the following drawback when the object is moving toward or away from the camera. Namely, when the amount of defocus is detected by one focus detecting operation and the focusing lens is moved to the in-focus position based on the amount of defocus, the object image is not in the in-focus state actually, since the object moves during the operation.
FIG. 13 is a graph illustrating the above described state. The abscissa represents time and the ordinate represents the amount of defocus on the film surface.
In the figure, the curve 1 represents the increase of the defocus amount on the film surface when the object moves toward the camera, and the line m is a trace of positions on which images are formed by the photographic lens. The central points A, B, C . . . of the time of accumulation are the time points at which the data of the object are read in. The first central point of accumulation is represented by T.sub.0 in FIG. 13. The central point of accumulation means the central time point of the time period required for CCD accumulation for determining the focus state. The amount of defocus at this time is represented by D.sub.0. The time period from T.sub.0 to T.sub.1 is the time period from the central point of accumulation to the end of accumulation and a time period required for the focus detecting operation. The time T.sub.1 to T.sub.2 is the time period for driving the lens. When the driving of the lens is completed, the lens is stopped and the next accumulation (T.sub.2 to T.sub.3) and the next operation (T.sub.3 to T.sub.4) are carried out. At the time point when the lens is stopped, the object has already moved and therefore there is a defocus of (D.sub.1 -D.sub.0) as compared with the time T.sub.0. The data of the object is taken again at the time period T.sub.3, the amount of defocus (D.sub.2 -D.sub.0) is calculated, and the driving of the lens is completed at the time period T.sub.5. As this time, the object image has already moved, so that even if the lens is driven (T.sub.5), there is a further amount of defocus (D.sub.3 -D.sub.2), which amount of defocus is larger than that at the point T.sub.2 . In the similar manner, the amount of defocus increases as the time passes (T.sub.8, D.sub.5 -D.sub.4), (T.sub.11, D.sub.7 -D.sub.6), going away from the in-focus state. Namely, although the AF operation has been carried out, the amount of defocus increases continuously, disabling releasing in the in-focus state.
The delay in tracking in the AF control becomes a serious problem especially when an interchangeable lens having longer focal length such as telephoto lens is used, since the focusing speed is slow.
In a so called one shot AF camera, in which the driving of the lens is stopped after the in-focus state is once obtained (AF lock), the amount of defocus increases continuously as the object moves until the shutter releasing operation is carried out (the time period is not constant), so that automatic focus adjustment cannot be carried out for a moving object.